Implement uring in AsyncWrite for fs::File

[RedKinda]

Motivation

Currently IO operations for fs::File is implemented using spawn_blocking to perform IO, which is not great for an async runtime. This PR implements IO operations using io_uring. The fs::write function already uses io_uring for async operations, and this PR extends the uring functionality into impl AsyncWrite for fs::File.

For full discussion see here #7684 and subsequently on discord https://discord.com/channels/500028886025895936/810724255046172692/1427735341003051079

Solution

The fs::File struct already implements an internal buffer when writing using spawn_blocking, which is similar to what io_uring requires. Therefore in poll_write, we can check if io_uring is initialized and supported, and if not fall back to the spawn_blocking implementation, without requiring any API changes.

This PR only implements AsyncWrite, and once this PR is accepted, I am happy to also open a PR for AsyncRead for File using the same structure.

[martin-g]

Does the Future need to be Sync ?
I think it is not really necessary.

[RedKinda]

Looks like it does need Sync, at least its required by a test https://github.com/RedKinda/tokio/blob/uring_file/tokio/tests/async_send_sync.rs#L651 - https://github.com/tokio-rs/tokio/actions/runs/18909744941/job/53976898682?pr=7713#step:6:248

[Darksonn]

In principle you could avoid Sync using src/util/sync_wrapper.rs. But for this use-case, it does not matter.

[martin-g]

I think there is a chance that the buffer is not fully drained here.
Write(Ok()) should be returned only if buf.is_empty().

I think you need to wrap the write in a loop and handle it with something like:

Ok(0) => break (Operation::Write(Err(io::ErrorKind::WriteZero.into())), buf),
Ok(_) if buf.is_empty() => break (Operation::Write(Ok(())), buf),
Ok(_) => continue, // more to write
Err(e) => break (Operation::Write(Err(e)), buf),

[RedKinda]

Good catch, thanks and fixed

[martin-g]

Same as above - make sure the buf is empty before saying Ok().